1. Technical Field
This invention relates to a retention canceling mechanism for a chain tensioner for canceling the retained state of the chain tensioner at the time of actually mounting the chain tensioner shipped in this state.
2. Related Art
It is known that a chain installed under tension between chain sprockets of a camshaft and a crankshaft, such as an engine timing chain, is subjected to oscillations during rotation if the chain has any slack. For preventing this, a chain tensioner is employed for thrusting the chain for perpetually affording a pre-set tension thereto. Such chain tensioner, described in JP Utility Model Kokoku Publication JP-B-58-38189, includes a housing mounted as a main body portion at a pre-set location, a cylinder bored within the housing, a sleeve slidably fitted within the cylinder, a spring mounted under compression within the sleeve for thrusting one end of the sleeve against the chain, a rotary plate having its one end rotatably mounted on the housing and having its opposite end engaged with a pin protruded towards the sleeve by a hook for retaining the sleeve for prohibiting the sleeve from being protruded under the force of the spring, a tooth shape formed on the outer periphery of the sleeve and a pawl member pivotally mounted by a hinge on the housing. The pawl member is biased by the spring into meshing with the tooth shape for inhibiting only the receding movement of the sleeve. The hook on the sleeve side portion of the rotary plate is formed with a sloped portion gradually raised towards the opposite end. When the sleeve is receded as it is thrust, the sleeve side pin is moved along the sloped portion to rotate the rotary plate for disengaging the rotary plate from the pin thereby canceling its retention. That is, the sleeve has to be transiently receded for canceling the state of retention between the rotary plate and the sleeve pin. The tooth shape is formed with play for canceling the state of retention and the sleeve can be receded axially by a length corresponding to the length of the play.
However, according to the investigation by the present inventors, it has turned out that there is the following problem. Namely, when actually mounting the chain tensioner with the above construction, an excess tension is applied to the chain by a value corresponding to the length of the play for thrusting the sleeve for canceling the state of retention. The result is that the chain is actuated under an excess tension corresponding to the restoration (push-back) of the thrust chain.